Crystal oscillators for surface mounting using a surface-mount type container for surface mounting and accommodating a crystal blank and an IC (integrated circuit) chip, which has an oscillation circuit using this crystal blank, in the container configured as a single unit are small and light, and are therefore widely used especially as frequency and time reference sources in portable electronic devices represented by cellular phones.
As one type of crystal oscillator for surface mounting, there is one using a container body having an H-figured cross section, on both principal surfaces of which recesses are formed, accommodating a crystal blank in the one recess and accommodating an IC chip on the other recess. FIGS. 1A and 1B are a cross-sectional view and a bottom view, respectively, showing the configuration of such a conventional crystal oscillator for surface mounting.
The crystal oscillator for surface mounting illustrated here is provided with container body 1 made of, for example, laminated ceramics. On one principal surface of container body 1, first recess 1a is formed, and on the other principal surface of container body 1, second recess 1b is formed. In container body 1, crystal blank 2 is accommodated in first recess 1a and IC chip 3 is accommodated in second recess 1b. Crystal blank 2 is, for example, an AT-cut quartz crystal blank, on both principal surfaces of which excitation electrodes (not shown) are formed, respectively. In crystal blank 2, lead-out electrodes extend from a pair of excitation electrodes toward both sides of one end of crystal blank 2, respectively. Both sides of one end of crystal blank 2 to which the lead-out electrodes extend are fixed to a pair of crystal holding terminals 4 provided on the bottom surface of first recess 1a of container body 1 using conductive adhesive 5. This causes crystal blank 2 to be held horizontally within recess 1a. Metal ring 6 is provided at an opened end face of recess 1a and by bonding metal cover 7 to metal ring 6 through seam welding, crystal blank 2 is hermetically encapsulated within first recess 1a. 
IC chip 3 is made up of electronic circuits integrated on a semiconductor substrate, the electronic circuits including an oscillation circuit using crystal blank 2. On the bottom surface of second recess 1b of container body 1, there are provided a plurality of mounting electrodes 9 to which a plurality of IC terminals 8 of IC chip 3 are fixed, respectively, and IC chip 3 is fixed to the bottom surface of second recess 1b through flip-chip bonding which bonds corresponding IC terminals 8 and mounting electrodes 9 together by means of bumps 10. In IC chip 3, since electronic circuits are formed on one principal surface of the semiconductor substrate, this principal surface is called a “circuit forming surface” of IC chip 3.
The plurality of IC terminals 8 are provided on the circuit forming surface which is the surface on which IC chip 3 is bonded to container body 1. These IC terminals 8 include: a pair of crystal terminals 8a and 8b used for electrical connection between IC chip 3 and crystal blank 2; a power supply terminal; an output terminal; a grounding terminal; and an AFC (automatic frequency control) terminal. In the example shown here, there are further provided two independent writing terminals as IC terminals used to write temperature compensation data into IC chip 3. These IC terminals 8 are electrically connected to the electronic circuits within IC chip 3 and are provided along a pair of mutually facing sides, for example, in the lengthwise direction on the substantially rectangular circuit forming surface. Especially, crystal terminals 8a and 8b are connected to the oscillation circuit within IC chip 3 and in this way, an oscillation closed loop made up of the oscillation circuit and crystal blank 2 is formed. Of IC terminals 8, the power supply terminal and grounding terminal are used to supply electric power to the oscillation circuit and the AFC terminal is used to supply an AFC signal to the oscillation circuit. Oscillation output fout from the oscillation circuit appears at the output terminal in IC chip 3. For explanations, FIG. 1B shows the outline of IC chip 3 with a dotted line and also shows IC terminals 8 and mounting electrodes 9 hidden by IC chip 3 and further shows monitoring electrodes 12a and 12b which will be described later. Therefore, the dotted line in FIG. 1B shows a region in second recess 1b in which IC chip 3 is placed.
Mounting electrodes 9 are all formed as substantially rectangular, flat-shaped electrode layers and are formed facing respective IC terminals 8 of IC chip 3 and arranged in parallel along the longitudinal direction of substantially rectangular second recess 1b of container body 1. In the figure, four mounting electrodes 9 are arranged on a row near each long side of recess 1b. Of these mounting electrodes 9, mounting electrodes 9a and 9b to be bonded to crystal terminals 8a and 8b by means of bumps 10 are electrically connected to a pair of crystal holding terminals 4 provided on the bottom surface of first recess 1a via conductive paths including via holes (not shown) provided in container body 1. Other mounting electrodes 9 connected to IC terminals 8 such as the power supply terminal, output terminal and grounding terminal are electrically connected to external terminals 11 which are provided in the four corners of the end face surrounding second recess 1b of container body 1, that is, the outer bottom surface of container body 1. External terminals 11 are the terminals used to surface-mount this crystal oscillator for surface mounting on a wiring board. Furthermore, mounting electrodes 9 connected to the writing terminals are electrically connected to external terminals (not shown) for data writing provided on the outer side surface of container body 1.
In addition to mounting electrodes 9, a pair of monitoring electrodes 12a and 12b used for characteristic testing of crystal blank 2 as a crystal element are also provided on the inner bottom surface of second recess 1b of container body 1. Monitoring electrodes 12a and 12b extend from mounting electrodes 9a and 9b connected to crystal terminals 8a and 8b on IC chip 3 side and are provided in a central region of the inner bottom surface of second recess 1b, that is, within the region to be covered with IC chip 3. In the figure, of mounting electrodes 9 arranged in two rows, second mounting electrodes 9 counted from the positions of both ends on one side of a diagonal of second recess 1b for the respective rows of mounting electrodes 9 are mounting electrodes 9a and 9b to be connected to crystal terminals 8a and 8b, respectively. These two mounting electrodes 9a and 9b and monitoring electrodes 12a and 12b are electrically connected by elongated conductive patterns 12c and 12d formed on the bottom surface of recess 1b. Monitoring electrodes 12a and 12b are substantially rectangular and have the areas greater than the areas of mounting electrodes 9, 9a and 9b to make it easier to contact a probe of a jig connected to a measuring instrument (not shown) with monitoring electrodes 12a and 12b. 
Monitoring electrodes 12a and 12b are used to judge defective products in the manufacturing process of crystal oscillators. More specifically, after hermetically encapsulating crystal blank 2 within first recess 1a of container body 1 and before accommodating IC chip 3 in second recess 1b, the probe is made to contact monitoring electrodes 12a and 12b to thereby test the oscillation characteristic of crystal blank 2 as a crystal element and separate conforming products from non-conforming products. IC chips are mounted on only products judged as conforming products in a later process.
Furthermore, monitoring electrodes 12a and 12b can also be used to adjust the vibration frequency of crystal blank 2. In such a case, before bonding metal cover 7 to metal ring 6, in the state that crystal blank 2 is fixed to crystal holding terminals 4 and, for example, the oscillation frequency of crystal blank 2 is adjusted by irradiating an ion beam onto crystal blank 2 while monitoring the vibration frequency of crystal blank 2 via monitoring electrodes 12a and 12b. 
In order to protect the circuit forming surface of the IC chip, protective resin is generally injected, as so-called underfill, between the IC chip and the bottom surface of the recess in which the IC chip is accommodated in a crystal oscillator in a configuration with the crystal blank and IC chip accommodated in different recesses of the container body.
Japanese Patent Laid-Open No. 2000-49560 (JP, 2000-049560, A) describes that monitoring electrodes are provided only within a region in which an IC chip is placed, that is, at a position below the IC chip, the monitoring electrodes are thereby electrically shielded by a grounding potential plane within the IC chip and it is thereby possible to prevent stray capacitance from being generated between the wiring pattern of the wiring board on which a crystal oscillator for surface mounting is mounted and the crystal blank.
In the crystal oscillator for surface mounting in the above described configuration, on the bottom surface of second recess 1b, monitoring electrodes 12a and 12b are drawn through conductive patterns 12c and 12d from mounting electrodes 9a and 9b, which correspond to crystal terminals 8a and 8b of IC chip 3, and provided within the region in which IC chip 3 is placed, that is, in the region between the two rows of mounting electrodes 9. IC chip 3 is fixed to the bottom surface of second recess 1b through flip-chip bonding in such a way that the circuit forming surface thereof faces monitoring electrodes 12a and 12b. 
As a result, stray capacitance C01 is generated between the circuit forming surface of IC chip 3, particularly a high-frequency circuit part such as the oscillation circuit in IC chip 3, and monitoring electrodes 12a and 12b, as shown in FIG. 2. In this case, the value of stray capacitance C01 is added up with a contribution of the extending path from mounting electrode 5, that is, conductive patterns 12c and 12d to monitoring electrodes 12a and 12b, and a contribution of the areas of mounting electrodes 9a and 9b. If the areas of conductive patterns 12c and 12d are sufficiently smaller than the areas of the electrodes, stray capacitance C01 results from the total area of monitoring electrodes 12a and 12b and mounting electrodes 9a and 9b. 
Furthermore, on the bottom surface of second recess 1b of container body 1, there are a plurality of mounting terminals 9 in addition to mounting electrodes 9a and 9b to which crystal terminals 8a and 8b are bonded, and each of monitoring electrodes 12a and 12b is surrounded by three of these mounting electrodes 9 on the bottom surface of second recess 1b. Since these mounting electrodes 9 other than mounting electrodes 9a and 9b are also electrically connected to the circuits in IC chip 3, stray capacitance C01 is further added up with a contribution of stray capacitance produced between monitoring electrodes 12a and 12b and these mounting electrodes 9. Moreover, stray capacitance C02 is also produced between monitoring electrodes 12a and 12b. 
Therefore, as shown in FIG. 2, a closed loop without routing through the crystal element (i.e., crystal blank 2) is formed with stray capacitances C01 and C02 independently of the original oscillation closed loop made up of the crystal element and oscillation circuit within IC chip 3. FIG. 2 shows such an additional closed loop with dotted lines. As a result, a high-frequency current i′ is produced which passes through the additional closed loop but not through the crystal element, and this current i′ is mixed up with oscillation current i in the original oscillation closed loop and outputted from output terminal fout of IC chip 3. The high-frequency signal outputted in such a way is liable to contain noise. Monitoring electrodes 12a and 12b accompanying stray capacitances C01 and C02 can thus constitute factors of deteriorating an operating characteristic of the crystal oscillator, for example, phase noise characteristic.
On the bottom surface of second recess 1b, monitoring electrodes 12a and 12b are provided in the region in which IC chip 3 is placed independently of mounting electrodes 9a and 9b, and therefore the sizes of monitoring electrodes 12a and 12b are restricted depending on the size of IC chip 3. As the crystal oscillator for surface mounting becomes more compact, the size of IC chip 3 is also reduced, and therefore the areas of monitoring electrodes 12a and 12b need to be reduced, too. For example, as for the planer outside size of the crystal oscillator, an already established standard size of 2.5×2.0 mm or 2.0×1.6 mm is available and in such a case, the inside size of the bottom surfaces of the recesses of the container body are on the order of 1.85×1.45 mm or 1.45×1.20 mm. In this way, as the size of the crystal oscillator is reduced, the areas of monitoring electrodes 12a and 12b are also reduced, and it becomes more difficult to make the probe touch or contact monitoring electrodes 12a and 12b. 